Method of manufacturing shadow masks

ABSTRACT

A shadow mask to be utilized in a color cathode ray tube is manufactured by the steps of preparing a shadow mask blank including laminated layers of a first metal and a second metal having different melting points, forming a plurality of small perforations through the shadow mask blank such that each perforation has a first predetermined diameter at the junction between the layers and a second predetermined diameter in the layer of the second metal, the second diameter being smaller than the first diameter and corresponding to the diameter of phosphor dots which are formed on a phosphor surface when the perforated shadow mask blank is used as a mask pattern for forming the phosphor dots by conventional photo-etching techniques and subjecting the perforated shadow mask to heat treatment for melting the layer of the second metal to enlarge the second diameter.

United States Patent A Tomita et al.

[ METHOD OF MANUFACTURING SHADOW MASKS [75] Inventors: Yoshifumi Tomita,Mobara;

Masamoto Akeyama, Kokubunzi, both of Japan 73 Assignee: Hitachi, Ltd.,Tokyo, Japan [22] Filed: May 6,1971

21 Appl.No.:140,917

[30] Foreign Application Priority Data May 6, 1970 Japan 45/38484 [52]U.S. Cl 29/25.11, 313/85 S [51] Int. Cl. HOlj 9/14 [58] Field of Search.156/3; 29/25.l3, 25.18, 25.17, 29/25.1l, 25.1; 313/85 S [56] ReferencesCited UNITED STATES PATENTS 3,631,576 l/1972 Law 29/25.l3 3,359,19212/1967 Heinrich et al. 204/143 1' Jan. 29, 1974 Primary ExaminerCharlesW. Lanham Assistant Examiner-J. W. Davie Attorney, Agent, or Firm-Craig,Antonelli & Hill 5 7] ABSTRACT A shadow mask to be utilized in a colorcathode ray tube is manufactured by the steps of preparing a shadow maskblank including laminated layers of a first metal and a second metalhaving different melting points, forming a plurality of smallperforations through the shadow mask blank such that each perforationhas a first predetermined diameter at the junction between the layersand a second predetermined diameter in the layer of the second metal,the second diameter being smaller than the first diameter andcorresponding to the diameter of phosphor dots which are formed on aphosphor surface when the perforated shadow mask blank is used as a maskpattern for forming the phosphor dots by conventional photo-etchingtechniques and subjecting the perforated shadow mask to heat treatmentfor melting the layer of the second metal to enlarge the seconddiameter.

6 Claims, 6 Drawing Figures PAIENTED 2 9 i974 INVENTORS YOSHIFUMITOP'IITA and NASAHOTO AKEYANA BY CmlgflMhmQQLw QELQQ ATTORNEYS METHOD OFMANUFACTURING SHADOW MASKS BACKGROUND OF THE INVENTION This inventionrelates to a method of manufacturing shadow masks for use in shadow masktype color cathode ray tubes.

A shadow mask type color cathode ray tube generally comprises threeelectron guns mounted in one end of a glass envelope, a shadow maskprovided with a plurality of small perforations positioned adjacenttheface plate of the envelope, and a phosphor surface having a mosaic ofa plurality of phosphor dots corresponding to the small perforations onthe face plate. A triad of phosphor dots adapted to emit three differentlight colors is formed on the phosphor surface at a position tocorrespond to one of the small perforations of the shadow mask and thesetriads are excited by electron beams passing through the perforations inthe mask for emitting different colors, whereby a picture image ofdesired color is reproduced. Thus, the shadow mask acts as a means forshaping the electron beams so that they will impinge upon the triads ofphosphor dots corresponding to the respective electron beams fordifferent colors. More particularly, as shown in FIG. 1 of theaccompanying drawings, a metal plate 1 is formed with a plurality ofsmall perforations 3 having a definite spacing for passing electronbeams 2. The configuration of each perforation takes the form of afrustum of a cone having a larger diameter on the side of the phosphorsurface than on the side of the electron guns (not shown) in order toprevent irregular reflection of the electron beams on the inner wall ofthe perforation obliquely impinging upon it.

It is also known that, in addition to the ability to shape the colorelectron beam, as described above, the shadow mask is also used as amask pattern for forming the phosphor dots on the phosphor surface ofthe color cathode ray tube using the conventional photo-etchingtechnique.

In recent years, the so-called matrix type color cathode ray tube hasbeen developed wherein the regions between the phosphor dots are coatedwith a black substance, graphite for example, in order to absorb ambientlight. It has also been proposed to make the diameter of the phosphordot smaller than that of the electron beam so as to improve the colorpurity, thus improving contrast. Where the shadow mask is used as themask pattern in such a color cathode ray tube, it is necessary to firstform small perforations through the shadow mask for forming the phosphordots using the photoetching technique and then enlarge the diameter ofthe perforations for shaping the electron beams.

For this reason, it has been the practice to first form smallperforations through the shadow mask using the etching process, saidperforations having a diameter smaller than the diameter of the electronbeam. After using the perforated shadow mask as the mask pattern, thediameter of the small perforations is enlarged by the etching process toa diameter sufficient to obtain an electron beam having a desireddiameter larger than the phosphor dot, thus forming a second perforationfor passing the electron beam. In such a case, it is important to shapethe inner wall of the second perforation as a frustum of a cone, withthe result that it is difficult to etch the perforation by the secondetching process so as to provide the'inner wall of uniformconfiguration. Further, there is a problem that during the etchingprocess, the etching solution also etches the mask frame, which isprovided for supporting the shadow mask, or the leaf springs welded tothe frame, thus causing an undesirable peeling off of the welds. Thus,the prior method of manufacturing the shadow mask is complicated becauseit requires two etching steps, one

, for forming small perforations to enable the shadow mask to be used asthe mask pattern for forming the phosphor dots and the other forenlarging the first perforation to have a diameter of a size sufficientfor passing the electron beam.

SUMMARY OF THE INVENTION Accordingly, a first object of this inventionis to provide a method of manufacturing a shadow mask according to whichit is possible to prepare by a single etching step a shadow mask forshaping electron beams to have diameters larger than those of thephosphor dots.

A second object is to provide a method of forming a shadow maskcomprising the steps of forming small perforations which serve as a maskpattern by etching from opposite surfaces a multi-layer metal plateconsisting of a first metal layer and a second metal layer, formingphosphor dots by a well known light exposure technique, and then heatingthe mask pattern to enlarge the diameter of the small perforationspenetrating through the second metal layer.

A third object of this invention is to provide a novel method ofmanufacturing a shadow mask from a blank consisting of metal layers ofdifferent melting points.

A fourth object of this invention is to provide a shadow mask havingsmall openings for shaping electron beams having diameters larger thanthose of the phosphor dots at the junction between two laminated metallayers of different melting points.

BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings:

FIG. 1 shows an enlarged perspective view of a portion of a conventionalshadow mask for use in a color cathode ray tube;

FIG. 2 shows an enlarged section of a portion of one example of a shadowmask blank utilized to manufacture the novel shadow mask for use in acolor cathode ray tube;

FIGS. 3 to 5 show enlarged sections of perforations formed by usingvarious types of etching solutions; and

FIG. 6 is an enlarged sectional view of a portion of a finished shadowmask manufactured according to the method of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT According to this invention, ashadow mask blank consisting of a multi-layer metal plate is prepared bybonding together a first metal layer, to be located in the tube envelopefacing the phosphor layer, and a second metal layer having a lowermelting point than the first metal layer, to be located facing theelectron guns, and small perforations are formed through the blank. The

. configuration of each perforation is selected such that it has adiameter corresponding to the desired electron beam diameterat thejunction between the two metal layers;'while, in the first metal layerthe diameter of the perforation gradually increases from the junctiontoward the surface facing the phosphor surface and in the second metallayer the diameter of the perforation is smaller than the desireddiameter at least at a portion thereof. The diameter of the perforationsin the second metal layer is selected to determine the diameter of thephosphor dots on the phosphor surface which are formed when theperforated shadow mask blank is used as a mask pattern for forming thephosphor dots by a photo-etching technique. Then the shadow mask blankis heat treated to melt the perforations through the second metal layerto cause an enlarging of the perforations through the second metallayer.

With reference now to FIG. 2 of the accompanying drawing, which shows across-section of a shadow mask blank utilized in this invention, theshadow mask blank comprises a first metal layer 4 and a second metallayer 5, having a lower melting point than the first metal layer andbeing laminated thereon. Bonding of the first and second metal layers isaccomplished by plating, or spraying or rolling, the second metal layeronto the first metal layer. In this embodiment, the first metal layer 4comprises an iron sheet having a thickness of about 100 microns and thesecond metal layer comprises a film formed by a plating of molten tinand having a thickness of about 50 microns. A plurality of smallperforations are formed through the shadow mask blank by well knownetching technique utilizing an etching solution. As shown by theenlarged sectional views of FIGS. 3 to 5, the configuration of each oneof the small perforations is selected such that it has a diameter Dcorresponding to the desired beam diameter at the junction 45 betweenthe iron sheet 4 and the tin layer 5; while, the diameter in the ironsheet 4 gradually increases from D at the junction 45 towards thesurface 41, thus forming a frustum of a cone, and the diameter in thetin layer 5 gradually decreases from D at the junction 45 towards thesurface 51 to reach a minimum at least at a portion of the perforation.The difference in the configurations of the perforations shown in FIGS.3 to 5, which depends upon the selection of the etching solution, iscaused by the difference between the solubilities of the iron sheet 4and the tin film 5 in the etching solution. More particularly, thecondition shown in FIG. 3 corresponds to a larger solubility of the tinfilm 5 than that of the iron sheet 4; that shown in FIG. 4 correspondsto an equal solubility of both metals; and that shown in FIG. 5corresponds to a larger solubility of the iron sheet 4 than that of thetin film 5. The minimum diameter d of the perforations through the tinfilm 5 formed by etching of the shadow mask blank is dependent upon themagnitude of the phosphor dots formed by the photo-exposure describedlater.

The shadow mask blank perforated as above described is used as anexposure mask pattern for depositing a black coating and phosphor dotson the phosphor surface. In this case, the tin film 5 of the shadow maskblank is located to face a light source for exposure, whereas the ironsheet 4 is located to face the phosphor surface.

The shadow mask blank, which has been used as the mask pattern, is thensubjected to heat treatment. The temperature of the heat treatment ishigher than the melting point (232C) of tin but lower than the meltingpoint of iron. As a consequence the tin layer 5 of the shadow mask blankmelts with the result that the surface tension of molten tin eliminatesthe projection that has been defining the minimum diameter d of theperforations shown in FIGS. 3 to 5, thuschanging the crosssectionalconfiguration of the perforations to that shown in FIG. 6. Thus, theheat treatment of the shadow mask blank produces a shadow mask havingperforations of the minimum diameterD that corresponds to the desiredelectron beam diameter and has been formed at the junction by theprevious etching process.

As above-mentioned, since the temperature of heat treatment is lowerthan the melting point of iron, the inner wall of-the perforationsthrough the iron sheet 4 formed by the etching operation is neverdeformed.

Although in the foregoing embodiment the first and second metal layersof shadow mask blank were comprised by iron and tin, respectively, it isto be understood that the invention is never limited to these particularmetals and that any metals having different melting points can also beused. Further, one or both of the metal layers may be a metal alloy.

While we have shown and described several embodiments in accordance withthe present invention, it is understood that the same is not limitedthereto but is susceptible of numerous changes and modifications asknown to a person skilled in the art, and we therefore do not wish to belimited to the details shown and described herein but intend to coverall such changes and modifications as are obvious to one of ordinaryskill in the art.

We claim:

1. A method of manufacturing a shadow mask for use in a color cathoderay tube having a phosphor surface formed of a mosaic of phosphor dotscomprising the steps of preparing a shadow mask blank includinglaminated layers of iron and tin having different melting points, thelayer of tin having a thickness of about microns, forming a plurality ofsmall perforations through said shadow mask blank such that each of saidperforations has a firstpredetermined diameter at the junction betweensaid iron and tin layers and a second predetermined diameter within thelayer-of tin, said second diameter being smaller than said firstdiameter, forming said phosphor dots through said perforated shadow maskblank by a photo-exposure technique with the diameter of said phosphordots being determined by said second diameter, and then subjecting saidperforated shadow mask blank to heat treatment for melting the layer oftin to enlarge said second diameter of said perforations.

2. The method according to claim 1, wherein said small perforations areformed such that the diameter of each perforation through said layer ofsaid first metal gradually increases from said junction to the oppositesurface of said layer.

3. The method according to claim 1, wherein said first predetermineddiameter of said perforations is dependent upon the diameter of anelectron beam that passes through said perforations and said seconddiameter is dependent upon the diameter of said phosphor dots.

4. The method according to claim 1, wherein said shadow mask blank isprepared by applying a coating of second metal on the layer of saidfirst metal.

5. The method according to claim 1, wherein said shadow mask blank isprepared by plating the layer of

1. A method of manufacturing a shadow mask for use in a color cathoderay tube having a phosphor surface formed of a mosaic of phosphor dotscomprising the steps of preparing a shadow mask blank includinglaminated layers of iron and tin having different melting points, thelayer of tin having a thickness of about 50 microns, forming a pluralityof small perforations through said shadow mask blank such that each ofsaid perforations has a first predetermined diameter at the junctionbetween said iron and tin layers and a second predetermined diameterwithin the layer of tin, said second diameter being smaller than saidfirst diameter, forming said phosphor dots through said perforatedshadow mask blank by a photo-exposure technique with the diameter ofsaid phosphor dots being determined by said second diameter, and thensubjecting said perforated shadow mask blank to heat treatment formelting the layer of tin to enlarge said second diameter of saidperforations.
 2. The method according to claim 1, wherein said smallperforations are formed such that the diameter of each perforationthrough said layer of said first metal gradually increases from saidjunction to the opposite surface of said layer.
 3. The method accordingto claim 1, wherein said first predetermined diameter of saidperforations is dependent upon the diameter of an electron beam thatpasses through said perforations and said second diameter is dependentupon the diameter of said phosphor dots.
 4. The method according toclaim 1, wherein said shadow mask blank is prepared by applying acoating of second metal on the layer of said first metal.
 5. The methodaccording to claim 1, wherein said shadow mask blank is prepared byplating the layer of said first metal with said second metal.
 6. Themethod according to claim 1, wherein said perforations are formed insaid shadow mask blank by a photo-etching process.